1. Field of the Invention
This invention relates to data communication methods, transmitters, and cellular radio communication systems, and more particularly, is suitable for use in a radio communication system such as a portable telephone system.
2. Description of the Related Art
In this kind of radio communication system, an area providing communication services is segmented into cells with a predetermined size. Base stations as stationary radio stations are deployed within the cells, respectively.
A portable telephone as a mobile radio station can communicate with the base station within the cell where the portable telephone exists. Thus, a so-called cellular system is constructed. For the cellular system, various systems have been proposed as the communication system between a portable telephone and a base station. As the representative one, there is a time division multiple access (TDMA) system.
In this TDMA system, as shown in FIGS. 1A and 1B, a predetermined frequency channel is segmented temporally into frames (F0, F1, . . . ) each having a predetermined time interval. Each frame is further divided into time slots TS0 to TS3, each time slot has a predetermined time interval. At the timing of time slot TS0 allocated to a user, the user uses the time slot to transmit a transmission signal. This system realizes a plurality of communications (so-called multiplex communication) by the same frequency channel and therefore utilizes frequency efficiently. Note that in the following description, time slot TS0 allocated for transmission is called transmission slot TX, while a block of data (i.e., data unit) that is sent with one transmission slot TX is called a slot.
Here, the transmitter and receiver of a radio communication system to transmit and receive data by taking an advantage of this TDMA system will be described with FIGS. 2 and 3. Incidentally, the transmitter and receiver shown in FIGS. 2 and 3 are mounted, for example, in the portable telephone and base station of a portable telephone system, respectively. The transmitter and the receiver are used in the communication from a portable telephone to a base station (so-called uplink communication) and the communication from a base station to a portable telephone (so-called downlink communication).
As shown in FIG. 2, the transmitter 1 is roughly constituted by a convolution coding circuit 2, an interleave buffer 3, a segmenting circuit 4, a modulation circuit 5, a pilot symbol adding circuit 6, a transmitting circuit 7, and an antenna 8. A data bit series S1 (transmission data) is first input to the convolution coding circuit 2.
The convolution coding circuit 2 consists of a predetermined stage-number of shift registers and exclusive OR circuits. The convolution coding circuit 2 performs convolution coding on the input data bit series S1 and then outputs the resulting coded bit series S2 to the interleave buffer 3. The interleave buffer 3 stores the coded bit series S2 in its internal storage region in order. If the coded bit series S2 is stored in the entire storage region (i.e., if the coded bit series S2 is accumulated by a desired amount), then the order of the coded bit series S2 will be randomly rearranged (rearranging this order will hereinafter be referred to as interleaving). The resulting coded bit series S3 is output to the segmenting circuit 4. Incidentally, the interleave buffer 3 has a storage capacity equivalent to a plurality of slots so that the coded bit series S3 is dispersed to a plurality of transmission slots TX.
The segmenting circuit 4 segments the coded bit series S3 at intervals of a predetermined number of bits in order to allocate the coded bit series S3 to transmission slots TX. The resulting coded bit group S4 is output to the modulation circuit 5 in order. The modulation circuit 5 performs a predetermined modulation process (e.g., a modulation process in a synchronous detection system such as QPSK modulation) on the supplied coded bit group S4 and then outputs the resulting data symbol group S5 to the pilot symbol adding circuit 6.
As shown in FIG. 4, the pilot symbol adding circuit 6 adds pilot symbols P as a header at the head of each symbol group (i.e., the head of data symbol I) of the data symbol group S5 segmented according to the transmission slots TX, and then outputs the resulting transmission group S6 to the transmitting circuit 7. Incidentally, the pilot symbols P added here are a known symbol pattern that has previously been known at the receiver side, and at the receiver side these pilot symbols P are employed to estimate the characteristics of the transmission path (e.g., fading, etc.).
The transmitting circuit 7 performs a filtering process on the transmission symbol group S6 added with these pilot symbols P in sequence and then performs a digital-to-analog conversion process on the transmission symbol group S6 to generate a transmission signal. And the transmitting circuit 7 performs a frequency transformation on the transmission signal, thereby generating a transmission signal S7 having a predetermined frequency channel. After this signal has been amplified to a predetermined electric power, it is transmitted through the antenna 8. In this manner, the transmission signal S7 is transmitted from the transmitter 1 in synchronization with the timing of the transmission slots TX.
On the other hand, as shown in FIG. 3, the receiver 10 is roughly constituted by an antenna 11, a receiving circuit 12, a transmission path estimating circuit 13, a demodulation circuit 14, a slot coupling circuit 15, a deinterleave buffer 16, and a Viterbi decoding circuit 17. The transmission signal S7 transmitted from the transmitter 1 is received by the antenna 11, and this is input to the receiving circuit 12 as a received signal S11.
The receiving circuit 12 amplifies the input received signal S11 and then performs a frequency transformation on the received signal S11, thereby taking out a base band signal. The receiving circuit 12 performs a filtering process on the base band signal and then performs an analog-to-digital. conversion process on the base band signal, thereby taking out a received symbol group S12 corresponding to the transmission symbol group S6. The received symbol group S12 is output to the transmission path estimating circuit 13.
The transmission path estimating circuit 13 is one which investigates the characteristic of the transmission path and also performs an equivalent process according to the result of investigation. The transmission path estimating circuit 13 estimates the characteristic of the transmission path by making a reference to the pilot symbols P included in the received symbol group S12, and computes the inverted characteristic of the transmission path, based on the result of estimation. And the transmission path estimating circuit 13 convolution-multiples a numerical value, which indicates the inverted characteristic of the transmission path, and each data symbol portion of the received symbol group S12, by using an equalizing circuit consisting of an equalizer. With this multiplication, the influence of fading caused on the transmission path is removed. With this process, the transmission path estimating circuit 13 restores the transmitted data symbol group 85 and outputs this to the modulation circuit 14 as a received data symbol group S13.
The modulation circuit 14 performs a predetermined modulation process on the received data symbol group S13, thereby restoring the coded bit group S14 corresponding to the coded bit group S4 on the transmitter side. The coded bit group S14 is output to the slot coupling circuit 15. Incidentally, each bit in the coded bit group S14 is not a binary signal such as a logic 0 or a logic 1 but has become a multi-level signal because of a noise component added on the transmission path.
The slot coupling circuit 15 is one which couples the group S14 of separate coded bits obtained in a slot unit so that they become a series signal. If the coded bit group S14 is accumulated by an amount corresponding to the storage capacity of the deinterleave buffer 16 of the latter stage, then the coded bit group S14 will be coupled together. The resulting code bit series S15 is output to the deinterleave buffer 16.
The deinterleave buffer 16 has a storage capacity equivalent to a plurality of slots. The deinterleave buffer 16 stores the supplied coded bit series S15 in its internal storage region in series and then rearrange the order of the coded bit series S15 in the order opposite the rearrangement made by the interleave buffer 3 of the transmitter 1, thereby returning the arrangement to the original order (such returning to the original order will hereinafter be referred to as deinterleaving). The resulting coded bit series S16 is output to the Viterbi decoding circuit 17.
The Viterbi decoding circuit 17 consists of a soft judgment Viterbi decoding circuit, and estimates a maximum likelihood state from among all state transitions that can be obtained as data (maximum likelihood estimation), by using the trellis diagram of the convolution codes based on the input coded bit series S16. With this estimation, the transmitted data bit series S18 is restored and output.
Incidentally, in radio communication systems with such constitution, in the case where the transmission signal S7, for example, is transmitted through a single frequency channel by the transmitter 1 and where the transmission signal S7 and waveforms delayed due to the multipath are simultaneously received by the receiver, the waveforms with time delay will overlap with the transmission signal S7, and consequently, frequency-selective fading will take place. Since this overlapped portion causes intersymbol interference, data cannot be accurately restored.
Hence, the receiver 10 takes an advantage of the fact that the symbols within each slot are temporally arranged and transmitted, and performs a convolution multiplication process on the time domain by an equalizing circuit consisting of an equalizer so that the influence of frequency-selective fading is removed. For this reason, there is a problem that the constitution of the receiver becomes complicated.
In view of the foregoing, an object of this invention is to provide a data communication method, a transmitter, and a cellular radio communication system which are capable of transmitting data while reducing the influence of fading caused on the transmission path.
The foregoing object and other objects of the invention have been achieved by the provision of a data transmitting method which transmits communication data by using a plurality of subcarriers arranged within a predetermined frequency width. In the data transmitting method, a data block consisting of the plurality of subcarriers arranged in rows in a time axis direction is used as a data unit when the communication data is transmitted, and both a differential modulation process based on each phase difference in a frequency axis direction between the plurality of subcarriers and a differential modulation process based on each phase difference in the time axis direction between the plurality of subcarriers are implemented, whereby a transmission signal that symbol data of the communication data are superposed on each phase difference between the plurality of subcarriers is generated and output.
Further, according to this invention, in a data receiving method which receives communication data transmitted by using a plurality of subcarriers arranged within a predetermined frequency width, a data block consisting of the plurality of subcarriers arranged in rows in a time axis direction is used as a data unit when the communication data is transmitted, and both a differential modulation process based on each phase difference in a frequency axis direction between the plurality of subcarriers and a differential modulation process based on each phase difference in the time axis direction between the plurality of subcarriers are implemented, whereby a transmission signal that symbol data of the communication data are superposed on each phase difference between the plurality of subcarriers is received. Differential demodulation processes based on each phase difference in the frequency axis, and time axis directions are implemented on the received signal, whereby the symbol data superposed on each phase difference between the plurality of subcarriers are demodulated and the communication data are restored.
Further, according to this invention, in a data transmitter which transmits communication data by using, a plurality of subcarriers arranged within a predetermined frequency width, modulating means, which uses a data block consisting of a plurality of subcarriers arranged in rows in a time axis direction as a data unit when the communication data is transmitted, for implementing both a differential modulation process based on each phase difference in a frequency axis direction between the plurality of subcarriers and a differential modulation process based on each phase difference in the time axis direction between the plurality of subcarriers, whereby a transmission signal that symbol data of the communication data are superposed on each phase difference between the plurality of subcarriers is generated. transmitting means for transmitting the transmission signal through a predetermined frequency channel.
Further, according to this invention, in a data receiver which receives communication data transmitted by using a plurality of subcarriers arranged within a predetermined frequency width, receiving means, which uses a data block consisting of the plurality of subcarriers arranged in rows in a time axis direction as a data unit when the communication data is transmitted, for implementing both a differential modulation process based on each phase difference in a frequency axis direction between the plurality of subcarriers and a differential modulation process based on each phase difference in the time axis direction between the plurality of subcarriers, whereby a transmission signal that symbol data of the communication data are superposed on each phase difference between the plurality of subcarriers is received. demodulating means for implementing differential demodulation processes based on each phase difference in the frequency axis and time axis directions on the received signal received by the receiving means, whereby the symbol data superposed on each phase difference between the plurality of subcarriers are demodulated and the communication data is restored.
Furthermore, in a cellular radio communication system, a predetermined area is segmented into cells with a predetermined size, then a base station is deployed in each of the cells, and a mobile station communicates communication data with the base station where that mobile station exists. At the mobile station, a data block consisting of a plurality of subcarriers arranged in rows in a time axis direction is used as a data unit when the communication data is transmitted, and both a differential modulation process based on each phase difference in the frequency axis direction between the plurality of subcarriers and a differential modulation process based on each phase difference in the time axis direction between the plurality of subcarriers are implemented, whereby a transmission signal that symbol data of the communication data are superposed on each phase difference between the plurality of subcarriers is generated and output. At the base station, the transmission signal is received, and differential demodulation processes in the frequency axis and time axis directions are implemented on the received signal, whereby the symbol data superposed on each phase difference between the plurality of subcarriers are demodulated and the communication data is restored.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.